Brake pressure control apparatus

ABSTRACT

A brake pressure control apparatus comprises a master cylinder (2), an auxiliary hydraulic power source (10), a booster (3), a comparator member (30), an inlet valve (28), a pressure raising member (31) and a limiter member (32). The comparator member (30) determines whether a fluid pressure in a fluid path connecting a boost chamber of the booster (3) with wheel brakes (8 and 9) is lower than a predetermined value. The inlet valve (28) cuts off flow of the fluid toward the wheel brakes (8 and 9) when it is determined that the fluid pressure in the fluid path connecting the boost chamber with the wheel brakes (8 and 9) is lower than the predetermined value. The pressure raising member (31) raises a brake pressure to be applied to the wheel brakes (8 and 9), based on a fluid pressure from the master cylinder. The limiter member (32) limits operation of the pressure raising member (31). Thus, the pressure raising member (31) is enabled to operate only when the fluid pressure in the fluid path between the boost chamber and the inlet valve (28) is lower than the predetermined value at the time of cutoff by the inlet valve (28).

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a brake pressure control apparatus forcontrolling a brake pressure for wheel brakes of a vehicle.

2. Description of the Prior Art

Well-known apparatus for brake pressure control adopt a system ofantilock control or traction control for wheel brakes of a vehicle usingan auxiliary hydraulic power source for constantly keeping a highpressure to be applied as an auxiliary hydraulic pressure. It is alsoknown to use a hydraulic booster making use of an auxiliary hydraulicpressure from an auxiliary hydraulic power source in place of aconventional vacuum type booster. Such a hydraulic booster can be easilymade to be small-sized and to have a high multiplying factor and it canbe combined with a master cylinder and an antilock control device toform a unitary body, which can be easily attached to a vehicle.

It is known that in the case of using a hydraulic booster, a system isadopted in which pressure is applied to a specified wheel brake by usinga dynamic fluid having a boosted pressure created in a boost chamber ofthe hydraulic booster, not using a static fluid through a mastercylinder as in other conventional systems. For example, Japanese PatentLaying-Open No. 104449/1982 discloses a pressure control mechanism of abrake operating cylinder, in which an antilock device associated withthe dynamic fluid comprises an intake valve to be operated forcommunication between a boost chamber and a specified wheel brake, and adischarge valve for discharging the dynamic fluid from the wheel braketo a reservoir. In such a manner, the master cylinder does not movebeyond a specified distance in the stroke in a repressurization processof antilock, as is different from the case of using the static fluid andthus the antilock device for the dynamic fluid can be constructed in avery simplified manner.

The brake system using the conventional vacuum type booster involves adisadvantage that a leg-power needs to be increased when a failureoccurs in a front wheel brake circuit to which the greater part of thebrake force is applied. In order to eliminate the disadvantage, a brakesystem using a hydraulic booster as described above uses a tandem typemaster cylinder, for example, in the case of a front-engine front-wheeldrive car, in which pressures of the two chambers of the master cylinderare applied to the respective front wheel brakes and the pressurethrough the booster is applied to the rear wheel brakes. In the case ofa front-engine rear-wheel drive car, a single type master cylinder isused, so that the pressure through the master cylinder is applied to thefront wheel brakes and the pressure through the booster is applied tothe rear wheel brakes.

However, in the above described brake system using the hydraulicbooster, it becomes impossible to apply the pressure to the rear wheelbrakes when the auxiliary hydraulic power is no longer applied as aresult of a failure in the auxiliary hydraulic power source, or when afailure occurs in the rear wheel brake circuit. In such a case, theboosting function of the booster is lost and a leg-power necessary for asufficient working of the brakes should be increased. As a result,vehicles to which the above described brake system using the hydraulicbooster is applicable are limited to those having light weight.

To solve the above described problems, it may be considered to use amethod for ensuring the boosting function by cutoff of a fluid pathbetween the boost chamber and the rear wheel brakes when a pressureswitch for monitoring the pressure of the auxiliary hydraulic powersource detects lowering of the auxiliary hydraulic pressure due to afailure in the rear wheel brake circuit. However, even in such a method,it becomes impossible to apply pressure to the rear wheel brakes, when afailure occurs in the auxiliary hydraulic power source itself and thebooster is not operated normally.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a brakepressure control apparatus which can solve the above describeddifficulties.

The brake pressure control apparatus according to the present inventioncomprises:

a master cylinder driven by a leg-power applied to a brake pedal, forapplying pressure to a first wheel brake;

an auxiliary hydraulic power source for sucking a fluid from a reservoirand storing it constantly under a prescribed pressure to be applied asan auxiliary hydraulic pressure;

a booster having a boost chamber, for connecting the boost chamber withthe reservoir when the brake pedal is not operated, and for receivingthe auxiliary hydraulic pressure from the auxiliary hydraulic powersource when the brake pedal is operated, causing the pressure to beboosted in the boost chamber in proportion to a leg-power, whereby theboosted pressure is applied to increase a thrust of the master cylindercaused by the leg-power and it is also applied to a second wheel brake;

comparator means having one end connected to a fluid path communicatingwith the master cylinder and the other end connected to a fluid pathbetween the boost chamber and the second wheel brake, to compare fluidpressures at both ends, so that it is determined whether the fluidpressure in the fluid path between the boost chamber and the secondwheel brake is lower than a predetermined value;

cutoff means disposed in the fluid path between the boost chamber andthe second wheel brake, for cutting off flow of the fluid from the boostchamber to the second wheel brake when it is determined that the fluidpressure in the fluid path between the boost chamber and the secondwheel brake is lower than the predetermined value;

pressure raising means having one end connected to a fluid pathcommunicating with the master cylinder and the other end connected to afluid path between the cutoff means and the second wheel brake, forraising the brake pressure to be applied to the second wheel brake basedon the pressure from the master cylinder when it moves toward the otherend upon receipt of the pressure from the master cylinder at the oneend; and

limiter means for limiting operation of the pressure raising means toenable the pressure raising means to be operated only when the flow ofthe fluid is cutoff by the cutoff means and the fluid pressure in thefluid path between the boost chamber and the cutoff means is lower thanthe predetermined value.

When the fluid pressure in the fluid path for between the boost chamberand the second wheel brake becomes lower than the predetermined value,the comparator means detects the state. Then, the cutoff means cuts offthe flow of the fluid from the boost chamber to the second wheel brake.

Accordingly, when a failure occurs in the second wheel brake downstreamof the cutoff means, the boosted pressure can be immediately regained bythe cutoff of the cutoff means and the booster can perform the normalboosting function. Thus, insufficiency of the leg-power can be avoided.

On the other hand, when a failure occurs on the side upstream of thecutoff means, that is, in the boost chamber, the following operation isperformed. When a failure occurs in the boost chamber, the fluidpressure in the fluid path in the boost chamber is still lower than thepredetermined value even after the cutoff by the cutoff means. In thiscase, the limiter means is in a state permitting operation of thepressure from raising means. Accordingly, the pressure raising meansreceives the pressure from the master cylinder at the one end thereofand moves toward the other end, whereby the brake pressure to be appliedto the second wheel brake is raised based on the pressure of the mastercylinder. Thus, the brakes of all the wheels are effectively operatedand the pressure for assuring a predetermined working of the brakes issubstantially the same as in the normal operation. In other words, evenif the booster is incapable of performing the boosting function,increase of the leg-power is never required.

Thus, according to the present invention, increase of the leg-power isnever required if a failure occurs in any portion of the apparatus,irrespective of whether it is upstream or downstream of the cutoffmeans. Therefore, no limitation is imposed to the weight of a vehicle towhich the present invention is to be applied.

These objects and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a hydraulic circuit diagram of a first embodiment of thepresent invention.

FIG. 2 is a hydraulic circuit diagram of a second embodiment of thepresent invention.

FIG. 3 is a hydraulic circuit diagram of a third embodiment of thepresent invention.

FIGS. 4 and 5 are enlarged sectional views of a piston 41 and a casing42 used in the third embodiment. Particularly, FIG. 4 shows the piston41 in a stop position and FIG. 5 shows the piston 41 in an operationposition.

FIG. 6 is a hydraulic circuit diagram of a fourth embodiment of thepresent invention.

FIGS. 7 and 8 are enlarged sectional views showing a structure of aportion associated with a piston 54 and a casing 55 used in the fourthembodiment. Particularly, FIG. 7 shows the piston 54 in a stop positionand FIG. 8 shows the piston 54 in an operation position.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a hydraulic circuit diagram of the first embodiment of thepresent invention. A brake pressure control apparatus shown in FIG. 1comprises: a brake unit 1 including a tandem type master cylinder 2having two chambers, and a booster 3; an auxiliary hydraulic powersource 10 including a motor, a pump and an accumulator for sucking afluid from a reservoir 11 and storing it constantly under apredetermined pressure, which is to be applied as an auxiliary hydraulicpressure; pressure reduction regulating means 12 and 13 for antilockcontrol of front wheel brakes 5 and 6; and pressure reduction regulatingmeans 14 for antilock control of rear wheel brakes 8 and 9.

The master cylinder 2 is driven by a leg-power applied to a brake pedal100 so that the front wheel brakes 5 and 6 are pressurized. The booster3 has a boost chamber 3a and when the brake pedal 100 is operated, thebooster 3 receives the auxiliary hydraulic pressure from the auxiliaryhydraulic power source 10 to produce a boosted pressure in the boostchamber 3a in proportion to the leg-power. The boosted pressure isapplied to increase the thrust of the master cylinder 2 caused by theleg-power and it is also applied to the rear wheel brakes 8 and 9.

The pressure reduction regulating means 12 is provided in a fluid path4a connecting one of the chambers of the master cylinder 2 and one ofthe front wheel brakes, i.e., the brake 5. The pressure reductionregulating means 13 is provided in a fluid path 4b connecting the otherchamber of the master cylinder 2 and the other front wheel brake 6. Thepressure reduction regulating means 14 is provided in fluid paths 7, 7aand 7b connecting the boost chamber 3a of the booster 3 and the rearwheel brakes 8 and 9.

The auxiliary hydraulic pressure of the auxiliary hydraulic power source10 is transmitted to the booster 3 through fluid paths 15 and 15b and itis also transmitted to the pressure reduction regulating means 12 and 13on the front wheel side through fluid paths 15 and 15a. The fluiddischarged from the front-wheel pressure reduction regulating means 12and 13 returns to the reservoir 11 through fluid paths 16a and 16. Thefluid discharged from the rear-wheel pressure reduction regulating means14 returns to the reservoir 11 through fluid paths 16b and 16.

A proportional reduction valve 17 is provided in the fluid pathconnecting the boost chamber 3a of the booster 3 and the rear-wheelpressure reduction regulating means 14. The proportional reduction valve17 reduces an input fluid pressure in proportion to a predeterminedvalue and outputs the reduced pressure, so that brake force isdistributed approximately in an ideal proportion.

The front-wheel pressure reduction regulating means 12 on one sidecomprises a control chamber 18 having two opposing ends, the first onebeing connected to the fluid path communicating with the front wheelbrake 5 and the second one being connected to the fluid pathcommunicating with the auxiliary hydraulic power source 10 and thereservoir 11. It further comprises a pressure reduction piston 19provided movably in the control chamber 18, an inlet valve 20 providedin a fluid path transmitting the auxiliary hydraulic pressure from theauxiliary hydraulic power source 10 to the second end of the controlchamber 18, for regulating an input amount of the fluid into the controlchamber, an outlet valve 21 provided in a fluid path discharging thefluid from the second end of the control chamber 18 to the reservoir 11,for regulating an output amount of the fluid, and a pressure-responsiveselection valve 22 provided in a fluid path communicating the mastercylinder 2 with the first end of the control chamber 18. The pressurereduction piston 19 cuts off flow of the fluid between the two ends ofthe control chamber 18 and it controls reduction of the brake pressurefor the front wheel brake 5 when it moves according to change in thefluid pressure at the second end of the control chamber 18.

In the same manner, the front-wheel pressure reduction regulating means13 on the other side comprises a control chamber 23, a pressurereduction piston 24, an inlet valve 25, an outlet valve 26, and apressure-responsive selection valve 27. Those two front-wheel pressurereduction regulating means 12 and 13 have exactly the same-constructionand perform the same operation. Therefore, the following description ismade only as to the front-wheel pressure reduction regulating means 12on one side and description of the other front-wheel pressure reductionregulating means 13 is omitted.

The inlet valve 20 in this embodiment is a 2-port 2-position selectionvalve. When the inlet valve 20 is in a non-conductive state (in whichelectric power is not supplied), it is pushed by a spring and maintainedin a first position as shown in the figure. The inlet valve 20 in thefirst position permits flow of the fluid from the auxiliary hydraulicpower source 10 to the control chamber and forbids the flow in thereverse direction. When the inlet valve 20 is in a conductive state (inwhich electric power is supplied), it is switched to a second position.The inlet valve 20 in the second position forbids flow of the fluid inboth directions.

The outlet valve 21 in this embodiment is a 2-port 2-position selectionvalve. When it is in the non-conductive state, it is by a spring andmaintained in a first position as shown in the figure. The outlet valve21 in the first position forbids flow of the fluid in the twodirections. When the outlet valve 21 is in the conductive state, itpermits flow of the fluid from the control chamber 18 to the reservoir11.

The pressure-responsive selection valve 22 in this embodiment is also a2-port 2-position selection valve. The position of this valve is changeddependent on the pressure in the fluid path communicating with thesecond end of the control chamber 18. When the pressure is high, thepressure-responsive selection valve 22 is maintained in a first positionas shown in the figure in opposition to the force of the spring. Thevalve 22 in that state permits flow of the fluid in both of thedirections. On the other hand, when the pressure in the fluid pathcommunicating with the second end of the control chamber 18 is lowered,the valve 22 is pushed by the spring and switched to a second position.The valve 22 in that state permits flow of the fluid from the frontwheel brake 5 to the master cylinder 2 and forbids flow of the fluid inthe reverse direction.

The rear-wheel pressure reduction regulating means 14 comprises an inletvalve 28 and an outlet valve 29. The inlet valve 28 is provided in thefluid path connecting the booster 3 with the rear wheel brakes 8 and 9,while the outlet valve 29 is provided in the fluid path connecting therear wheel brakes 8 and 9 with the reservoir 11.

The inlet valve 28 in this embodiment is a 2-port 2-position selectionvalve. When it is in the non-conductive state, it is pushed by a springand maintained in a first position as shown in the figure. The valve 28in that state permits flow of the fluid from the booster 3 to the rearwheel brakes 8 and 9. When the inlet valve 28 is in the conductivestate, it is switched to a second position. The valve 28 in the secondposition permits flow of the fluid from the rear wheel brakes 8 and 9 tothe booster 3 and forbids flow of the fluid in the reverse direction.

The outlet valve 29 in this embodiment is also a 2-port 2-positionselection valve. When it is in the non-conductive state, it is pushed bya spring and maintained in a first position as shown in the figure. Thevalve 29 in that state forbids flow of the fluid in both of thedirections. When it is in the conductive state, it is switched to asecond position. The valve 29 in that state permits flow of the fluidfrom the rear wheel brakes 8 and 9 to the reservoir 11.

Now, operation of the whole apparatus will be described. In thefollowing, control of brake force for the front wheel brakes 5 and 6will be first described and then control of brake force for the rearwheel brakes 8 and 9 will be described, for convenience sake.

Referring to FIG. 1, when the brake pedal 100 is operated, the fluidpressure in the master cylinder 2 is increased. The increased fluidpressure is applied to the front wheel brake 6 through the fluid path 4aand the pressure-responsive selection valve 22 as well as to the frontwheel brake 6 through the fluid path 4b and the pressure-responsiveselection valve 27.

When locking of the front wheels is detected, the inlet valves 20 and 25are brought into the conductive state and the outlet valves 21 and 26are also brought into the conductive state. Then, the pressure reductionpistons 19 and 24 both move in the control chambers 18 and 23,respectively, to lower the fluid pressure in the fluid pathscommunicating with the front wheel brakes 5 and 6. When the fluidpressure at the second end of the control chamber 18 becomes lower thana predetermined pressure, the pressure-responsive selection valves 22and 27 are switched to the second positions to forbid flow of the fluidfrom the master cylinder 2 to the front wheel brakes 5 and 6. Thus, thepressure reduction pistons 19 and 24 are moved toward the second ends ofthe control chambers 18 and 23, respectively, to increase the volume ofand reduce the pressure of the system including the front wheel brakes 5and 6, whereby locking of the front wheels is avoided.

When the inlet valves 20 and 25 are in the conductive state and theoutlet valves 21 and 26 are in the non-conductive state, the pressurereduction piston 19 is stopped so that the brake pressure for the frontwheel brakes 5 and 6 is maintained at a constant value. Then, when theinlet valves 20 and 25 and the outlet valves 21 and 26 are all in thenon-conductive state, the apparatus is in a state permittingrepressurization.

Next, control of brake force in a dynamic system for the rear wheelbrakes 8 and 9 will be described.

In normal operation of the brake pedal 100, the boosted pressure throughthe booster 3 is applied to the rear wheel brakes 8 and 9 through thefluid path 7, the inlet valve 28 and the fluid paths 7a and 7b. Whenlocking of the rear wheels is detected, the inlet valve 28 and theoutlet valve 29 are both brought into the conductive state. In thatstate, application of the boosted pressure through the booster 3 isstopped by the inlet valve 28, while the fluid in the fluid pathcommunicating with the rear wheel brakes 8 and 9 is discharged to thereservoir 11 through the outlet valve 29 and the fluid paths 16b and 16.As a result, the brake pressure for the rear wheel brakes 8 and 9 islowered to avoid locking of the rear wheels.

When the inlet valve 28 is in the conductive state and the outlet valve29 is in the non-conductive state, the brake pressure is maintained at aconstant value. When the inlet valve 28 and the outlet valve 29 are bothbrought into the non-conductive state, the apparatus is in a statepermitting repressurization.

The above described brake pressure control apparatus further comprisescomparator means 30, pressure raising means 31 and limiter means 32 totake measures against abnormal lowering of the fluid pressure in thefluid path connecting the boost chamber 3a of the booster 3 with therear wheel brakes 8 and 9. The inlet valves 28 function as cutoff means.

In this embodiment, the comparator means 30 is a differential pistonhaving one end connected to the fluid path 4b communicating with themaster cylinder 2 and the other end connected to the fluid path 7connecting the boost chamber 3a with the rear wheel brakes 8 and 9. Whenthe fluid pressure in the fluid path connecting the boost chamber 3awith the rear wheel brakes 8 and 9 is lower than a predetermined value,the comparator means 30 moves toward the side of the above mentionedother end, that is, rightward in the figure. The comparator means 30 inthe form of the differential piston has a cam surface 33. The comparator30 is further provided with a pin 34 having one end in contact with thecam surface 33. When the comparator means 30 moves toward the side ofthe above mentioned other end, the pin 34 is pushed upward as it comesout of the cam surface 33. Thus, the movement of the comparator means 34is detected by the movement of the pin 34. In other words, abnormallowering of the fluid pressure in the fluid paths between the boostchamber 3a and the rear wheel brakes 8 and 9 is detected by the movementof the switch 34. The cutoff means, that is, the inlet valve 28 isbrought into the conductive state dependent on information of the switch34, whereby communication between the boost chamber 3a and the rearwheel brakes 8 and 9 can be cutoff.

The pressure raising means 31 in this embodiment is a piston having oneend connected to the fluid path 4b communicating with the mastercylinder 2 and the other end connected to the fluid paths 7a and 7bbetween the inlet valve 28 and the real wheel brakes 8 and 9. When thepressure raising means 31 receives the pressure from the master cylinderat the one end and moves toward the side of the other end, the fluid inthe fluid paths 7a and 7b communicating with the rear wheel brakes 8 and9 is pressurized. In other words, as a result of the movement of thepressure raising means 31 toward the side of the above mentioned otherend, the brake pressure to be applied to the rear wheel brakes 8 and 9is increased based on the pressure from the master cylinder.

The limiter means 32 in this embodiment is also a piston having one endconnected to the fluid path 4b communicating with the master cylinder 2and the other end connected to the fluid path 7 between the boostchamber 3a and the inlet valve 28. The limiter means 32 in the form ofthe piston moves to the side of the above mentioned other end, that is,upward in the figure when the fluid pressure in the fluid path 7 betweenthe boost chamber 3a and the inlet valve 28 is lower than apredetermined value. The limiter means 32 has a stopper 35 on the sideof the above mentioned one end, which is to be engaged with the pressureraising means 31 to forbid movement of the pressure raising means 31. Asshown, when the stopper 35 is engaged with the pressure raising means31, the pressure raising means 31 cannot be moved. If the limiter means32 moves upward from the position shown in the figure, the engagementbetween the stopper 35 and the pressure raising means 31 is cancelledand the pressure raising means 31 moves, toward the side of its otherend, that is, rightward in the figure.

Now, description will be made of operation in case of abnormal loweringof the fluid in the fluid paths 7, 7a and 7b connecting the boostchamber 3a of the booster 3 with the rear wheel brakes 8 and 9.

When the fluid pressure in the fluid paths 7, 7a and 7b connecting theboost chamber 3a and the rear wheel brakes 8 and 8 becomes lower than apredetermined value the comparator means 30 moves rightward in thefigure. The movement of the comparator means 30 is detected by theswitch 34 and thus abnormal lowering of the fluid pressure is detected.Then, the inlet valve 28 as the cut off means is brought into theconductive state to cut off flow of the fluid from the boost chamber 3ato the rear wheel brakes 8 and 9.

If a failure occurs on the side of the boost chamber 3athe fluidpressure in the fluid path 7 connecting the boost chamber 3a with theinlet valve 28 is still lower than the predetermined value.Consequently, the limiter means 32 moves toward the boost chamber 3a andthe engagement between the stopper 35 and the pressure raising means 31is cancelled. The pressure raising means 31 receives the pressure fromthe master cylinder at the above mentioned one end and moves toward theother end, thereby to pressurize the fluid in the fluid paths 7a and 7bconnecting the inlet valve 28 with the rear wheel brakes 8 and 9. Thus,even if a failure occurs in the boost chamber 3athe brake pressure forthe rear wheel brakes 8 and 9 can be increased based on the pressurefrom the master cylinder.

On the other hand, if a failure occurs on the side of the inlet valve 28and the rear wheel brakes 8 and 9, the fluid pressure in the fluid path7 connecting the boost chamber 3a with the inlet valve 28 is increasedto a normal value by cutoff of the inlet valve 28. Thus, the limitermeans 32 does not move upward in the figure and the engaging statebetween the stopper 35 and the pressure raising means 31 is maintained.Thus, the booster 3 performs the normal boosting function andinsufficiency of the leg-power does not occur.

FIG. 2 is a hydraulic circuit diagram of the second embodiment of thepresent invention. In the following description of the secondembodiment, only the features different from those of the firstembodiment will be explained.

In the first embodiment shown in FIG. 1, the master cylinder 2 is of thetandem type, while in the second embodiment, the master cylinder 2 is ofa single-chamber type. The comparator means and the limiter means areformed by a single piston 36. The pressure raising means 40 is in theform of a piston, as in the first embodiment. The piston 36 has a camsurface 37, with which an end of a switch 38 is to be in contact, in thesame manner as in the first embodiment. In addition, the piston 36 has astopper 39 to be engaged with the pressure raising means 40 to forbidmovement of the means 40, similarly to the first embodiment.

Description is now made of operation of the second embodiment. When thefluid pressure in the fluid path connecting the boost chamber 3a withthe rear wheel brake 8 and 9 becomes lower than a predetermined value,the piston 36 moves upward in the figure. The movement of the piston 36is detected by the switch 38 and the inlet valve 28 is brought into theconductive state. As a result, flow of the fluid from the boost chamber3a to the rear wheel brakes 8 and 9 is cut off. In this case, if afailure occurs on the side of the boost chamber 3athe piston 36 ismaintained in the above mentioned position. Accordingly, the engagementbetween the stopper 39 and the pressure raising means 40 is cancelled.Thus, the pressure raising means 40 receiving the pressure from themaster cylinder at one end thereof moves to the side of the other end,so that the brake pressure to be applied to the rear wheel brakes 8 and9 is increased based on the pressure from the master cylinder.

On the other hand, if a failure occurs on the side of the fluid pathsconnecting the inlet valve 28 with the rear wheel brakes 8 and 9, thefluid pressure in those fluid paths is raised to a normal valueimmediately after the inlet valve 28 is brought into the conductivestate. Thus, the piston 36 is maintained in the lower position in thefigure and the engagement between the stopper 39 and the pressureraising means 40 is maintained.

FIG. 3 is a hydraulic circuit diagram of the third embodiment of thepresent invention. In the description of this third embodiment also,only the features different from the above embodiments will beexplained.

In the third embodiment, the comparator means, the cutoff means, thepressure raising means and the limiter means are formed by a singlepiston 41. In the above described embodiments, the inlet valve 29 as thecutoff means is provided to cut off the fluid paths connecting the boostchamber 3a with the rear wheel brakes 8 and 9 when the fluid pressure inthose fluid paths is abnormally lowered. On the other hand, in the thirdembodiment of FIG. 3, the cutoff means for performing such functions isa valve provided between the piston 41 and a casing 42 for containingthe piston 41.

FIGS. 4 and 5 show details of the piston 41 and the casing 42. FIG. 4shows a state in which the piston 41 is in a stop position and FIG. 5shows a state in which the piston 41 is in an operation position.

The casing 42 has a first bore 42, a second bore 44 and a third bore 45as shown. In association therewith, the piston 41 has first liquid sealmeans 46, second liquid seal means 47 and third liquid seal means 48 formaintaining a liquid seal state with respect to the first bore 43, thesecond bore 44 and the third bore 45, respectively. The third liquidseal means 48 serves also as valve means. In FIGS. 4 and 5, the diameterof the first bore 43 is shown by A, that of the second bore 44 is shownby C and that of the third bore 45 is shown by B. The diameter C is thelargest.

Further, as shown, ports 50, 51, 52 and 53 are formed in the casing 42.The pressure from the master cylinder 2 is applied to the port 50. Theport 51 communicates with the reservoir 11. The port 52 communicateswith the rear wheel brakes 8 and 9. The port 53 communicates with theboost chamber 3a. The piston 41 is pushed toward the side of the mastercylinder 2 by a spring 49. When the piston 41 is in the stop position asshown in FIG. 4, the third liquid seal means 48 is located apart fromthe third bore 45. In that case, the boost chamber 3a is in a statecommunicating with the rear wheel brakes 8 and 9.

When the fluid pressure in the fluid paths connecting the boost chamber3a with the rear wheel brakes 8 and 9 is a normal value, the piston 41is maintained in the stop position shown in FIG. 4. In this state, therear wheel brakes 8 and 9 can be pressurized by the boosted pressurethrough the boost chamber 3a.

When the fluid pressure in the fluid paths connecting the boost chamber3a with the rear wheel brakes 8 and 9 becomes lower than a predeterminedvalue, the piston 41 moves rightward in FIG. 4. As a result, the liquidseal state is maintained between the third liquid seal means 48 and thethird bore 45 and flow of the fluid from the boost chamber 3a to therear wheel brakes 8 and 9 is cut off.

If a failure occurs in the rear wheel brake 8 or 9 in that state, theboosted pressure through the boost chamber 3a regains a predeterminedvalue. Therefore, if A=B, the movement of the piston 41 is stopped andthe leg-power is caused to regain a predetermined value by the boostingfunction.

On the other hand, if a failure occurs in the boost chamber 3athe piston41 further moves rightward in the figure due to the pushing action ofthe pressure from the master cylinder applied to the area correspondingto the diameter A. As a result, a closed circuit including the rearwheel brakes 8 and 9 is pressurized by a force based on a difference inthe areas corresponding to C - B. FIG. 5 shows the state in which thepiston 41 is moved to the right position.

FIG. 6 is a hydraulic circuit diagram of the fourth embodiment of thepresent invention. Also in the description of this embodiment, only thefeatures different from the above embodiments will be explained.

In the fourth embodiment of FIG. 6, the comparator means, the pressureraising means and the limiter means are formed by a single piston 54.The cutoff means is provided in a casing 55 containing the piston 54.

FIGS. 7 and 8 show details of a portion related with the piston 54 andthe casing 55. FIG. 7 shows a state in which the piston 54 is in a stopposition and FIG. 8 shows a state in which the piston 54 is in anoperation position.

The casing 55 has a first bore 56 of a diameter A, a second bore 57 of adiameter C and a third bore 58 of a diameter B. The piston 54 has firstliquid seal means 54, second liquid seal means 60 and third liquid sealmeans 61 for keeping a liquid seal state in association with the firstbore 56, the second bore 57 and the third bore 58, respectively. Thediameter C is the largest.

As shown, the casing 55 has a port 69 communicating with the mastercylinder 2, has a port 70 communicating with the reservoir 11, a port 71communicating with the rear wheel brakes 8 and 9, and a port 72communicating with the boost chamber 3a. In addition, as shown, a valvebody 62, a movable valve seat 63 and a sleeve 64 are incorporated in thepiston 54. A spring 65 is disposed between the valve body 62 and themovable valve seat 63 to constantly push the movable valve seat 63rightward in the figures. The sleeve 64 is fixed to the piston 54 toprevent the movable valve seat 63 from being moved out of the piston 54.A valve body 67 constantly pushed toward the piston 54 by a spring 68 iscontained in the third bore 58 of the casing 55. Further, a spring 66for constantly pushing the piston 54 toward the master cylinder iscontained in the third bore 58.

If the fluid pressure in the fluid paths connecting the boost chamber 3awith the rear wheel brakes 8 and 9 is a normal value, the piston 54 ismaintained in the stop position shown in FIG. 7. In that position, theboost chamber 3a is in a state communicating with the rear wheel brakes8 and 9. Thus, the rear wheel brakes 8 and 9 can be pressurized by theboosted pressure through the boost chamber 3a.

When the fluid pressure in the fluid paths connecting the boost chamber3a with the rear wheel brakes 8 and 9 becomes lower than a predeterminedvalue, the piston 54 moves rightward in the figures. As a result, thevalve body 67 and the movable valve seat 63 are brought into a closedstate and then the movable valve seat 63 and the valve body 62 arebrought into a closed state. Consequently, flow of the fluid of theboost chamber 3a to the rear wheel brakes 8 and 9 is cut off.

If a failure occurs in the rear wheel brake 8 or 9 in that state, theboosted pressure regains to a normal value. Therefore, if A=B, themovement of the piston 54 is stopped and the leg-power is caused toregain to a determined value by the boosting function.

On the other hand, if a failure occurs in the boost chamber 3athe piston54 moves rightward due to the pressure from the master cylinder appliedto an area corresponding to the diameter A. As a result, the closedcircuit including the rear wheel brakes 8 and 9 is pressurized by aforce based on a difference in the areas corresponding to C - B.

In the foregoing, the present invention has been described in connectionwith the four embodiments. However, those embodiments are chosen by wayof illustration and example only and are not be taken by way ofillustration. Therefore, various changes and modifications can be madewithout departing from the scope of the invention as defined by thefollowing claims.

What is claimed is:
 1. A brake pressure control apparatus comprising:amaster cylinder driven by a leg-power applied to a brake pedal, forstoring a pressure to a first wheel brake, an auxiliary hydraulic powersource for sucking a fluid from a reservoir and storing the fluidconstantly under a pressure, to be applied as an auxiliary hydraulicpressure, a booster having a boost chamber, connecting said boostchamber to said reservoir when said brake pedal is not operated, andproducing a boosted pressure in said boost chamber in proportion to theleg-power upon receipt of the auxiliary hydraulic pressure from saidauxiliary hydraulic power source when said brake pedal is operated toenable the boosted pressure to increase thrust of said master cylindercaused by the leg-power and to pressurize a second wheel brake,comparator means having one end connected to a fluid path communicatingwith said master cylinder and the other end connected to a fluid pathbetween said boost chamber and said second wheel brake, for comparinghydraulic pressures in both of the fluid paths to determine whether thefluid pressure in the fluid path between said boost chamber and saidsecond wheel brake is lower than a predetermined value, cutoff meansdisposed in the fluid path between said boost chamber and said secondwheel brake, for cutting off flow of the fluid from said boost chamberto said second wheel brake when it is determined that the fluid pressurein the fluid path between said boost chamber and said second wheel brakeis lower than the predetermined value, pressure raising means having oneend connected to the fluid path communicating with said master cylinderand the other end connected to a fluid path between said cutoff meansand said second wheel brake, for raising a brake pressure to be appliedto said second wheel brake based on a pressure from said master cylinderwhen it receives the pressure from said master cylinder at the one endand moves toward the other end, and limiter means for limiting operationof said pressure raising means to enable said pressure raising means tooperate only when the fluid pressure in the fluid path connecting saidboost chamber with said cutoff means is lower than the predeterminedvalue at the time of cutoff by said cutoff means.
 2. A brake pressurecontrol apparatus in accordance with claim 1, wherein said comparatormeans is a differential piston for receiving the pressure from saidmaster cylinder at the one end and the boosted pressure at the otherend.
 3. A brake pressure control apparatus in accordance with claim 2,wherein said differential piston has a cam surface, andmovement of saiddifferential piston is detected by movement of a switch having one endto be in contact with said cam surface.
 4. A brake pressure controlapparatus in accordance with claim 3, wherein said cutoff means is anelectromagnetic valve which permits flow of the fluid from said boostchamber to said second wheel brake when it is in a non-conductive stateand forbids the flow of the fluid from said boost chamber to said secondwheel brake when it is in a conductive state.
 5. A brake pressurecontrol apparatus in accordance with claim 3, wherein said cutoff meansis a valve provided between said differential piston and a casing forcontaining said differential piston.
 6. A brake pressure controlapparatus in accordance with claim 5, wherein said pressure raisingmeans is a piston.
 7. A brake pressure control apparatus in accordancewith claim 6, whereinsaid limiter means is a piston having one endconnected to the fluid path communicating with said master cylinder andthe other end connected to the fluid path between said boost chamber andsaid cutoff means, said limiter means moves toward the other end whenthe fluid pressure in the fluid path between said boost chamber and saidcutoff means is lower than the predetermined value, and said pressureraising means is enabled to operate only when said limiter means movestoward the other end.
 8. A brake pressure control apparatus inaccordance with claim 7, whereinsaid limiter means has a stopper at saidone end, which is to be engaged with said pressure raising means toforbid movement of said pressure raising means, and said limiter meansmoves toward said other end when the fluid pressure in the fluid pathbetween said boost chamber and said cutoff means is lower than thepredetermined value at the time of cutoff by said cutoff means, therebyto cancel the engagement between said stopper and said pressure raisingmeans.
 9. A brake pressure control apparatus in accordance with claim 1,wherein said comparator means and said limiter means are formed by asingle piston.
 10. A brake pressure control apparatus in accordance withclaim 1, wherein said comparator means, said pressure raising means andsaid limiter means are formed by a single piston.
 11. A brake pressurecontrol apparatus in accordance with claim 1, wherein said comparatormeans, said cutoff means, said pressure raising means and said limitermeans are provided within a single casing.